Slotted cable glide slope antenna

ABSTRACT

This invention relates to antennas for use in glide slope systems for the instrument landing of airplanes. The antenna disclosed here may be used, as a phase-compensating element, in an end-fire glide slope array, to control the slope angle over a desired azimuth sector. A coaxial transmission line, fed from one end, has a number of gaps in the outer conductor which leak current onto the outside surface of the line. The separation between gaps varies along the line, being chosen to give a predetermined phase distribution of the radiation current. Tuning to an assigned frequency channel may be accomplished by filling the line to a specified pressure with a gas of relatively high dielectric constant.

United States Patent Watts, Jr.

[54] SLOTTED CABLE GLIDE SLOPE ANTENNA [72] Inventor: Chester B. Watts,Jr., 6505 Pinecrest Court, Annadale, Va. 22003 22 Filed: Dec. 16, 1070211 Appl. No.: 98,594

[52] US. Cl. "343/739, 343/771, 343/853 [51] Int. Cl. ..H01q 13/10 [58]Field of Search ..343/767, 771, 739, 853

[56] References Cited UNITED STATES PATENTS 2,408,435 10/1946 1 Mason..343/771 2,512,468 6/1950 Percival ..343/77l 3,577,197 5/1971 Watts..343/771 [451 Oct. 17, 1972 Primary Examiner Eli Lieberman [57]ABSTRACT This invention relates to antennas for use in glide slopesystems for the instrument landing of airplanes. The antenna disclosedhere may be used, as a phasecompensating element, in an end-fire glideslope array, to control the slope angle over a desired azimuth sector. Acoaxial transmission line, fed from one end, has a number of gaps in theouter conductor which leak current onto the outside surface of the line.The separation between gaps varies along the line,pbeing chosen to givea predetermined phase distribution of the radiation current. Tuning toan assigned frequency channel may be accomplished by filling the line toa specified pressure with a gas of relatively high dielectric constant.

7 5 Claims, 4 Drawing Figures SLOTTED CABLE GLIDE SLOPE ANTENNA Thisantenna is structurally similar to the one described in my co-pendingapplication Ser. No. 855,142 filed Sept. 4, 1969, titled Slotted CableLocalizer Antenna" except for the fact that the gap spacings arenon-uniform, and the antenna is fed at only one end.

HISTORY In 1942, soon after the appearance of equi-signal image-typeglide slope systems, Kandoian, U. S. Pat.

No. 2,367,680, proposed an end-fire glide slope array,

citing the hazard of the tall pole required for the imagetype systems.No solution was, however, presented for the primary difficulty with thisarrangement, namely, the high undesirable transverse shape. The shape ofthe glide slope surface, which follows directly from the geometry ofpath lengths associated with any simple end-fire array, is, ofnecessity, a narrow cone, with axis horizontal, coinciding with thephysical axis of the array. In following such a glide slope surface, anairplane could not deviate in azimuth from this axis without receivingan extraneous fly-down signal. The multilobe pattern, which is producedby the variation in path difference versus the angle off-axis, is, infact, a figure of revolution about the axis, the effect of azimuth beingindistinguishable from that of elevation. What was clearly needed was asimple antenna element to use in the end-fire array which would cancelor compensate for the effects of path difference due to azimuth angle,while leaving the elevation performance unchanged.

SUMMARY It is an object of this invention to provide a phasecompensatingantenna, which may be used as an element of an end-fire glide slopearray to control the glide slope angle, maintaining it relativelyconstant ver sus azimuth angle, over a specified azimuth sector. Theantenna is a coaxial transmission line, the outside surface of whichacts as a linear current radiator. Current is fed onto the outer surfaceby means of numerous shunted gaps in the outer conductor. The spacingsbetween the gaps are chosen to produce the desired current phasedistribution, while the gap shunts control the current amplitudedistribution. Although these desired relationships hold, in principle,for only a single r.f. frequency, the antenna is readily tunable over anarrow. range of frequencies, such as the glide slope band, by fillingthe line with a controlled amount of a gas such as sulfur hexafluoridehaving a relatively high dielectric constant.

LIST OF FIGURES FIG. 1 is a view, partially cut away, of an embodimentof the slotted cable antenna.

FIG. 2 is a graph of two design parameters; one being the excess of gapspacing over guide wavelength, and the other being the resultant phasedistribution.

FIG. 3a is a plan view of a runway layout, illustrating the use of twoslotted cable antennas as phase-compensating elements in an end-fireglide slope array.

FIG. 3b is an elevation view of the same end-fire glide slope array.

DESCRIPTION A satisfactory embodiment of the slotted cable antenna isillustrated in FIG. 1. This construction employs a length oftransmission line 2, of the rigid type, standard to the radiobroadcasting industry. Inner conductor 4 is supported concentric withoutouter conductor 6, typically by means of insulating pins 8. Connector 10provides the means for joining one end of the line to a source of r.f.energy. The opposite end of the line is terminated in a resistive load12. Placed at non-equal intervals along the line are numerous gapassemblies of which 14, 16, and 18 are typical. Gap assembly 16 is showncut-away to reveal, at the center of the assembly, the gap in the outerconductor 6. Inner conductor 4 passes through without interruption. Thegap is surrounded by metal sleeve 20 which is supported and separatedfrom the outer conductor 6 by means typified by insulators 22 and 24.Sleeve 20 thus acts as a capacitive shunt across the gap, the shuntingimpedance depending upon the sleeve length L and the relative diametersof sleeve 20 and outer conductors 6. If the length L is too long interms of the wavelength for the shunt to be considered as a lumpedcapacitor, it can be computed as a pair of transmission linetransformers of length L/2 each. In either case, the shunting impedancewhich is presented across the gap is made to be quite low compared tothe line impedance. Insulators 22 and 24 may include sealing means sothat any gas pressure existing within the transmission line will beretained.

Consider now what happens when r.f. energy is introduced into the linethrough connector 10. The energy propagates freely down the line,passing, in succession, the various gap assemblies typified by l4, l6,and 18. Since, in each case, the gap is shunted by a relatively lowimpedance, the main line current is only slightly affected.Nevertheless, a small fraction of the total current does escape to flowon the outside of the outer conductor in each case, resulting inradiation. The field produced some distance from the antenna is the cumulative effect of the leakage from all of the gaps. The remaining mainline current is absorbed in a matched termination, load 12.

The amplitude distribution of the radiation current is controlled byvarying the gap shunt impedances from end to end of the antenna, thelower the impedance in each case, the less the current that leaks out.

The phase distribution of the radiation current is controlled by thespacing between the gap assemblies. The main line current phase anglerotates through 360 with each guide-wavelength of distance along theline. Thus, if the: gaps were spaced exactly one guidewavelength apart,the various gap radiation currents would be in phase with one another.If, however, the gap spacing is somewhat greater than theguidewavelength, then the phase of the succeeding gap radiation currentlags thephase of the earlier gap radiation current. Also, the converseis true. Utilizing these facts, it is possible to generate anyreasonable shape of phase distribution.

FIG. 2 illustrates graphically the: relation between the parameters ofgap spacing and radiation current phase distribution. Curve 26 indicatesthat the first gaps have a spacing considerably greater than theguidewavelength, and that the spacing; gradually decreases toward thecenter of the antenna, where the spacing becomes equal to theguide-wavelength. Beyond the center, the spacing becomes progressivelyshorter than the guide-wavelength. This means that the early gaps lagtheir predecessors, while the later gaps lead their predecessors inphase angle. The phase distribution which results from this isapproximated by curve 28, which can be obtained numerically by summingall of the phase shifts from the beginning up to the gap in question.The desired shape, for the problem of the end-fire glide slope, iscircular, as will become apparent. It should be noted, however, that themethod is not limited to producing circular shapes.

FIG. 3a shows the approach end of a runway 30 in plan view. Suitablylocated alongside the runway are two slotted cable antennas servingrespectively as for ward element 32, and rear element 34 in an end-fireglide slope array. The antennas are fed by transmission lines 36 and 38which receive the usual carrier and sideband signals through bridgenetwork 40. Linestretcher 42 is provided as a means for adjusting therelative phases of the two antennas to produce the desired glide angle.Rear antenna 34 has the phase distribution 28 which was described inconnection with FIG. 2. Forward antenna 32 in FIG. 3a is identicalexcept turned end for end so that the closer spaced gaps are nearest thefed end. This has the effect of exchanging lead angles for lag angles,producing the reversed phase distribution 44. Both phase distributionsare designed to be circular. lf converted from phase angle to equivalentfree-space path length, each distribution curve would have a radius ofcurvature equal to D/2 where D is the front-to-rear separation, thecenter of curvature being midpoint 46. This means that when viewed froman airplane at low elevation angle in the direction of the central ray48, or anywhere in the azimuth sector between the extreme rays 50 and52, the forward and rear antennas appear to be radiating from the commonphase-center at midpoint 46. That is to say, there is no change versusazimuth, in the relative phase of the signals received from the twoantennas.

At the same time, viewed in elevation, FIG. 3b, the behavior isconventional, as from the two point sources. The path difference betweenthe two antennas 32 and 34 to a distant point, varies with elevationangle A, according to D cos A producing a multi-lobed pattern in theusual way. The main point is, however, that the elevation pattern,whatever it may be, remains unchanged with azimuth within the limitedsector controlled by the phase-compensating effect of the slotted cableantennas.

It will be understood that the desired relationship justed to the samevalue by filling the antenna with the proper amount of a high-dielectricgas such as sulfur hexafluoride, SP With rigid pin-supported air line,very nearly the entire space between the inner and outer conductors isavailable for filling with gas. In this case, the guide-wavelengthsimply varies inversely as the square-root of the dielectric constantwhich, in turn varies with the quantity, or pressure, of gas. Thus,

the dielectric constant needs to be controlled over a range of about 5percent, or from 1.00 to 1.05. This can be accomplished by filling withl to 17 atmospheres of SP8.

lclaim:

l. A slotted cable antenna comprising a coaxial transmission line,connector means at one end of said line for joining to a source of r.f.energy, a plurality of gaps in the outer conductor of said line,radiation control means disposed across each of said gaps, said controlmeans presenting in each case an impedance, whereby most of the linecurrent is conveyed across the gap, except for a portion of said currentwhich escapes to flow on the outside of said line, thereby producingradiation, and further, where the spacing of said gaps varies from endto end of said line, producing thereby a predetermined and non-linearphase distribution of the radiation current.

2. An antenna as in claim 1, with a resistive load connected to thesecond end of said line.

3. An antenna as in claim 1 with said radiation control means comprisinga metal tube, surrounding each of said gaps, but insulated therefrom.

4. An antenna as in claim 1, with frequency tuning means comprising aquantity of high-dielectric gas filling the space between inner andouter conductors of said coaxial transmission line.

5. An antenna as in claim 1, where the impedance presented by saidcontrol means varies from end to end of said line, producing thereby apredetermined amplitude distribution of the radiation current.

1. A slotted cable antenna comprising a coaxial transmission line,connector means at one end of said line for joining to a source of r.f.energy, a plurality of gaps in the outer conductor of said line,radiation control means disposed across each of said gaps, said controlmeans presenting in each case an impedance, whereby most of the linecurrent is conveyed across the gap, except for a portion of said currentwhich escapes to flow on the outside of said line, thereby producingradiation, and further, where the spacing of said gaps varies from endto end of said line, producing thereby a predetermined and nonlinearphase distribution of the radiation current.
 2. An antenna as in claim1, with a resistive load connected to the second end of said line.
 3. Anantenna as in claim 1 with said radiation control means comprising ametal tube, surrounding each of said gaps, but insulated therefrom. 4.An antenna as in claim 1, with frequency tuning means comprising aquantity of high-dielectric gas filling the space between inner andouter conductors of said coaxial transmission line.
 5. An antenna as inclaim 1, where the impedance presented by said control means varies fromend to end of said line, producing thereby a predetermined amplitudedistribution of the radiation current.